1. Field of the Invention
The present invention relates to an optical disc, an apparatus for reproducing information from the optical disc, and a method of recording information on and reproducing it from the optical disc.
2. Description of the Related Art
FIG. 9 shows a conventional apparatus for reading an optical disc
In FIG. 9, when music and image information is recorded on an optical disc 92, the information is converted into a signal, which parameter is a pit length of an information pit 91 on an information recording surface 95. When the optical disc 92 is read, the reproducing apparatus irradiates a laser light onto the information pit 91 to form a laser beam light spot LS having a predetermined diameter, detects the diffracted and reflected light by a light detector, which consists of a photodiode etc., and outputs it as an electric signal. Then, the reproducing apparatus extracts the recorded music and image information out of this electric signal by applying a signal conversion process contrary to the process at the time of recording, and outputs it. The compact disc (CD), the laser video disc (LVD) etc. are known as such an optical disc. In FIG. 9, a reference numeral 93 represents a substrate, which consists of polycarbonate resin etc., and a reference numeral 94 represents a protection layer.
As mentioned above, the signal is read out by use of the change in the reflected light quantity depending on the existence of the pit at the time of scanning the optical beam on the optical disc.
In such an optical disc, the information recording density is greatly influenced by the value of a track pitch P.sub.1 i.e. the interval between adjacent turns of the track each of which is a center line of the pit array of the information pit 91 as shown in FIG. 10, and the value of the diameter of the beam spot LS of the laser light irradiated onto the information pit 91.
Therefore, it may be proposed to narrow or reduce the value of the track pitch from the above explained value of FIG. 10, in order to record more information on the optical disc.
However, although there arises no problem in case of the examples shown in FIG. 9 and FIG. 10, if the track pitch is reduced to be the value P.sub.2 which is less than the value P.sub.1 of FIG. 10 (for example, P.sub.2 =P.sub.1 /2) as shown in FIG. 11, for example, information pits 91B and 91C on the neighboring tracks other than the track of the information pit 91A to be read, are included within the range of the beam spot LS of the laser light. Therefore, without any countermeasure, the crosstalk quantity increases, and it cannot be practically used. Thus, in order to prevent the signal from the neighboring track from leaking thereinto (crosstalk), the track pitch should be made wide enough with respect to the diameter of the beam spot LS, resulting in the obstacle to high density recording of the optical disc.
On the other hand, it may be also proposed that the diameter of the beam spot LS of the laser light is further reduced. However, as shown in FIG. 12, if the laser light with a wavelength .lambda. is condensed at the position of the focal length f by an objective lens OL, the minimum laser beam diameter w is expressed by a following expression (1). EQU W=1.22.times.(.lambda./NA) . . . (1)
Here, NA represents a quantity called a numerical aperture of the objective lens OL. If n represents the refraction coefficient of the objective lens, and .THETA. represents the output angle of the light from the objective lens, the numerical aperture NA is expressed by a following expression (2). EQU NA=n.times.sin.THETA.. . . (2)
Therefore, in order to reduce the minimum laser beam diameter W, the wavelength .lambda. may be reduced or the numerical aperture NA may be increased.
In addition, as for the wavelength .lambda. of the laser light, the wavelength .lambda. of the semiconductor laser presently available for optical discs, is about .lambda.=0.780 .mu.m (.mu.m (micrometer) is 10.sup.-6 m). Moreover, as for the value of the numerical aperture NA, it is about NA=0.45, in case of the CD. From this, the minimum laser beam diameter Wmin is approximately expressed by a following expression (3). EQU Wmin=1.22.times.(0.780/0.45)=2.1 .mu.m . . . (3)
Therefore, when the laser beam spot is irradiated on a certain pit array, the minimum track pitch which does not cause the crosstalk becomes about 1.6 .mu.m. This value is employed by many optical discs presently available.
On the other hand, there may be proposed a method of performing a high density record, in which two or more items of information are represented by one pit, and in which pattern recognition of one pit pattern itself is carried out. However, because of the pattern recognition of one pit pattern itself, this method should be poor to a positional offset of the pit pattern, and the constitution for pit-pattern detection should be complicated.